1 Introduction

The history of explanations in intelligent systems began with expert systems which were predominantly based on heuristics [7], but also on case-based reasoning (CBR) [12], and model based approaches [13]. In recent years their more commercial or entertainment inclined successors - recommender systems - have begun to offer explanations as well [5], [18], [24]. These systems represent user preferences for the purpose of suggesting items to purchase or examine, i.e. recommendations.